From db64fa259b9050c7eacc822a18a697e43176cadf Mon Sep 17 00:00:00 2001 From: Stuart Martin Date: Tue, 30 Apr 2002 19:35:33 +0000 Subject: [PATCH] desc & qs git-svn-id: svn://svn.rockbox.org/rockbox/trunk@344 a1c6a512-1295-4272-9138-f99709370657 --- firmware/test/wavey/README.playlists | 84 ++++++++++++++++++++++++++++ 1 file changed, 84 insertions(+) create mode 100644 firmware/test/wavey/README.playlists diff --git a/firmware/test/wavey/README.playlists b/firmware/test/wavey/README.playlists new file mode 100644 index 0000000000..fa28fd47a0 --- /dev/null +++ b/firmware/test/wavey/README.playlists @@ -0,0 +1,84 @@ + + Playlists on the Rockbox + +1. Demand-loading of Playlist Filenames from Disk + +A playlist is simply a list of track names. These lists can either be +created dynamically by the user, or they can be predefined and placed +into a text file with an .m3u extension. + +The 2048 KB of RAM is the reason we need to get this right. If an average +track name (i.e. \music\cure\disintegration\01-pictures_of_you.mp3) +is assumed to be 50 characters long, then: + +A playlist of 15 tracks is 15 * 50 ~= 750 bytes +A playlist of 100 tracks is 100 * 50 ~= 5 kilobytes +A playlist of 3500 tracks is 3500 * 50 ~= 175 kilobytes +A playlist of 10000 tracks is 10000 * 50 ~= 1/4 megabyte + +From these figures, it can be seen that for large playlists, storing +the entire list of track names in memory significantly reduces the +capacity available to the audio data buffer, which in turn has a negative +impact on the performance of the system. + +One method of reducing the total memory consumption of a playlist is +to delay bringing the actual filenames into memory until needed. Instead, +the playlist text file can be scanned, and an in-memory array constructed +with one element for each track present in the text file. Progressing +through the playlist entails getting the appropriate entry from the array, +and using that to perform a lookup of the corresponding filename entry +from the playlist text file. + +With this method, and given that an integer on the Rockbox's CPU is 4 bytes: + +A playlist of 15 tracks is 15 * 4 ~= 60 bytes +A playlist of 100 tracks is 100 * 4 ~= 400 bytes +A playlist of 3500 tracks is 3500 * 4 ~= 13 kilobytes +A playlist of 10000 tracks is 10000 * 4 ~= 39 kilobytes + +It is clear that these are substantial savings, albeit at the cost of +increased complexity and disk i/o. Prefetch strategies could improve +performance compared to demand-loading a single entry. + +2. Implementation Options + +Keeping the track names in a file on disk is easy enough for a predefined +m3u playlist, but for dynamically created playlists, where the user +browses the filesystem and adds tracks or entire directory hierarchies +at will, we will need to store the playlist track names in a dedicated +file. This will be called the Anonymous Playlist, the location of which +can be set by the user, but will default to \anonymous.m3u or somesuch. +The current contents of the Anonymous Playlist can be named and saved at +any time. + +The data structure to support playlists would therefore be: + +typedef struct +{ + char filename[256] ; /* path name of m3u playlist on disk */ + int *indices; /* array of indices into the playlist */ + int index; /* index of current track within playlist */ +} playlist_info_t; + +So far, so good: we read from an existing m3u file, or we create an +anonymous one. But what do we do if we start with an existing m3u file, +and then the user wants to dynamically add tracks to it? A few options +exist: + +a) we disallow playlist modification of existing m3u files, offering + instead to replace the current playlist with the new one. + +b) we give the user the option of appending the new tracks to the + existing m3u file. + +c) we copy the contents of the existing m3u playlist to the anonymous one, + and then append the new tracks to that. If the m3u playlist is large, + this could be wasteful and potentially time-consuming. However, choosing + this option would provide the facility to insert or append entire + existing m3u playlists 'into' one another, a feature missng from the + commercial firmware. + + + + +